The present invention relates in general to apparatus for feeding sheets seriatim from a stack, and more particularly to a sheet feed apparatus which substantially prevents image ruboff.
In typical commercial electrostatographic reproduction apparatus (copier/duplicators, printers, or the like), a latent image charge pattern corresponding to information to be reproduced is formed on a uniformly charged charge-retentive or photo-conductive member having dielectric characteristics (hereinafter referred to as the dielectric member). Pigmented marking particles are attracted to the latent image charge pattern to develop such image on the dielectric member. A receiver member is then brought into contact with the dielectric member, and an electric field applied to transfer the marking particle developed image to the receiver member from the dielectric member. After transfer, the receiver member bearing the transferred image is transported away from the dielectric member, and the image is fixed (fused) to the receiver member by heat and pressure to form a permanent reproduction thereon.
The rate at which such reproduction apparatus make copies can be quite significant (for example from seventy to one hundred thirty copies per minute). Such high copy rates are possible, at least in part, due to advances in feeding document sheets, bearing information to be reproduced, to and from a copy station. One sheet feeder which has been successful in reliably feeding sheets to and from the copy station is commonly referred to as an oscillating vacuum recirculating document feeder. Sheets are withdrawn seriatim, from a sheet stack supported in a tray, by a ported oscillating cylinder. The oscillating cylinder is selectively coupled to a vacuum source. When the ports of the oscillating cylinder are in juxtaposition with the sheet stack, the bottom-most sheet is vacuum tacked to the cylinder. The cylinder is then rotated in a direction to withdraw such sheet from the stack and feed the sheet into a travel path away from the sheet stack.
A pair of driven nip rollers are respectively associated with bearings supported on the oscillating cylinder. The nip rollers cooperate with the bearings to urge the withdrawn sheet in a downstream direction along the travel path. This cooperative arrangement enables a sheet to be transported along the travel path in the downstream direction substantially unimpeded by the oscillation of the oscillating cylinder. Once the sheet is in the nip between the nip rollers and the bearings so as to be under the transport control thereof, the oscillating cylinder can be rotated in the direction reverse to the first direction. Accordingly, the oscillating cylinder will rotate to return the ports to a position for withdrawing the next sheet from the sheet stack.
The described oscillating vacuum recirculating document feeder is very efficient in withdrawing sheets seriatim from the sheet stack. However, due at least in part to pressure points or zones existing between adjacent sheets in the sheet stack, when a sheet is withdrawn from the stack, the material used to form images on the sheets tends to rub off on adjacent sheets. Such image material can be pigmented marking particles or ink for example. Image ruboff can create significant markings on adjacent sheets which may then result in undesirable artifacts in images reproduced from information on the sheets. The problem is even more significant when the sheets are of the duplex type (i.e., information images exist on both sides of a sheet) since the ruboff can occur immediately over information to be reproduced.
The mentioned pressure points or zones causing the ruboff problems are the result of the weight of the sheet stack on zones of contact over the area supporting the sheet stack. For example, certain zones of contact correspond to ribs, oriented in the direction of the sheet travel path, for supporting the sheet stack. These ribs are intended to reduce overall friction contact of the bottom-most sheet and the tray supporting the sheet stack by decreasing the total contact area therebetween. Further, a zone of contact may correspond to the area, oriented in the direction in the plane of a sheet perpendicular to the sheet travel path, where the sheet stack is supported by the oscillating cylinder.